Re: [mm 4.15-rc8] Random oopses under memory pressure.

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Thu, Jan 18, 2018 at 03:58:30PM +0100, Andrea Arcangeli wrote:
> On Thu, Jan 18, 2018 at 06:45:00AM -0800, Dave Hansen wrote:
> > On 01/18/2018 04:25 AM, Kirill A. Shutemov wrote:
> > > [   10.084024] diff: -858690919
> > > [   10.084258] hpage_nr_pages: 1
> > > [   10.084386] check1: 0
> > > [   10.084478] check2: 0
> > ...
> > > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> > > index d22b84310f6d..57b4397f1ea5 100644
> > > --- a/mm/page_vma_mapped.c
> > > +++ b/mm/page_vma_mapped.c
> > > @@ -70,6 +70,14 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw)
> > >  		}
> > >  		if (pte_page(*pvmw->pte) < pvmw->page)
> > >  			return false;
> > > +
> > > +		if (pte_page(*pvmw->pte) - pvmw->page) {
> > > +			printk("diff: %d\n", pte_page(*pvmw->pte) - pvmw->page);
> > > +			printk("hpage_nr_pages: %d\n", hpage_nr_pages(pvmw->page));
> > > +			printk("check1: %d\n", pte_page(*pvmw->pte) - pvmw->page < 0);
> > > +			printk("check2: %d\n", pte_page(*pvmw->pte) - pvmw->page >= hpage_nr_pages(pvmw->page));
> > > +			BUG();
> > > +		}
> > 
> > This says that pte_page(*pvmw->pte) and pvmw->page are roughly 4GB away
> > from each other (858690919*4=0xccba559c0).  That's not the compiler
> > being wonky, it just means that the virtual addresses of the memory
> > sections are that far apart.
> > 
> > This won't happen when you have vmemmap or flatmem because the mem_map[]
> > is virtually contiguous and pointer arithmetic just works against all
> > 'struct page' pointers.  But with classic sparsemem, it doesn't.
> > 
> > You need to make sure that the PFNs are in the same section before you
> > can do the math that you want to do here.
> 
> Isn't it simply that pvmw->page isn't a page or pte_page(*pvmw->pte)
> isn't a page?
> 
> The distance cannot matter, MMU isn't involved, this is pure 64bit
> aritmetics, 1giga 1 terabyte, 48bits 5level pagetables are meaningless
> in this comparison.

Note, it's 32-bit.

> 
> #include <stdio.h>
> 
> int main()
> {
> 	volatile long i;
> 	struct x { char a[4000000000]; };
> 	for (i = 0; i < 4000000000*3; i += 4000000000) {
> 		printf("%ld\n", ((struct x *)0)-((((struct x *)i))));
> 	}
> 	printf("xxxx\n");
> 	for (i = 0; i < 4000000000; i += 1) {
> 		if (i==4)
> 			i = 4000000000;
> 		printf("%ld\n", ((struct x *)0)-((((struct x *)i))));
> 	}
> 	return 0;
> }
> 
> You need to add two debug checks on "pte_page(*pvmw->pte) % 64" and
> same for pvmw->page to find out the one of the two that isn't a page.
> 
> If both are real pages there's a bug that allocates page structs not
> naturally aligned.

Both are real page. But why do you expect pages to be 64-byte alinged?
Both are aligned to 64-bit as they suppose to be IIUC.

I can't say I fully grasp how 'diff' got this value and how it leads to both
checks being false.

[   10.209657] page:f6e4cc38 count:8 mapcount:6 mapping:f5818f94 index:0x0
[   10.209989] flags: 0x3501004d(locked|referenced|uptodate|active|mappedtodisk)
[   10.210496] raw: 3501004d f5818f94 00000000 00000005 00000008 00000100 00000200 00000000
[   10.210742] raw: f5c06600 00000000
[   10.210863] page dumped because: pvmw->page
[   10.210992] page->mem_cgroup:f5c06600
[   10.211192] page:f74749d8 count:1 mapcount:1 mapping:f54612d1 index:0x0
[   10.211381] flags: 0x15040068(uptodate|lru|active|swapbacked)
[   10.211530] raw: 15040068 f54612d1 00000000 00000000 00000001 f74749c4 f75b9014 00000000
[   10.211729] raw: f5c06600 00000000
[   10.211806] page dumped because: pte_page(*pvmw->pte)
[   10.211920] page->mem_cgroup:f5c06600
[   10.212079] diff: -858832092
[   10.212184] hpage_nr_pages: 1
[   10.212284] check1: 0
[   10.212384] check2: 0

-- 
 Kirill A. Shutemov

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